Electronic conduction and stability of solid electrolytes based on lanthanum gallates

Abstract

The electronic conductivity of some Sr- and Mg-doped lanthanum gallates was measured as a function of the oxygen activity in the p O 2 range of about 10 1–10 −17 bar using the Hebb–Wagner polarization technique. The investigated compositions were La 0.8Sr 0.2Ga 1− y Mg y O 3− δ with y=0.15, 0.2, (La 0.8Sr 0.2) 0.95Ga 0.8Mg 0.2O 3− δ (LSGM19/20), and the Nd-doped (La 0.9Nd 0.1) 0.8Sr 0.2Ga 0.8Mg 0.2O 3− δ (LNSGM). Sintered dense pellets of the samples were contacted with glass encapsulated ion-blocking Pt or Au microcontacts in pure N 2 atmosphere and a Cu 2O/CuO reference electrode. The temperatures ranged between 600 and 750 °C. For p O 2 >10 −8 bar, all samples showed p-type conductivity with the expected dependence σ h∝ p O 2 1/4 and a weak temperature influence. For p O 2 <10 −12 bar, n-type conduction was found which, however, did not give a corresponding σ e∝ p O 2 −1/4 dependence. Instead, the electronic conductivity approached a limiting value below about p O 2 =10 −15 bar. This result and the observation of an additional irreversible current drift after cathodic polarization for long times suggested a degradation of the materials in the low p O 2 range. This was most probably caused by the loss of Ga 2O and/or by the formation of PtGa x -alloys. Both effects can explain the observed limitation of the electron concentration in the n-type range.